Pressure sensitive recording paper

ABSTRACT

A pressure sensitive recording paper comprising a paper substrate and a coating thereon comprising a color former which comprises a member selected from the group consisting of acidtreated dioctahedral montmorillonite clay minerals and mixtures of such minerals with natural dioctahedral montmorillonite clay minerals, the minerals having a secondary color development property, K2, of at least 1.40, the value of K2 being represented by the formula

United States Patent [191 Sugahara et a1.

[ Aug. 21, 1973 PRESSURE SENSITIVE RECORDING PAPER [75] Inventors:Yojlro Sugahara, Tokyo; Kaichiro Mlyazawa, Tsuruoka; Tadahisa Nakazawa;Masahiro Maeno, both of Nakajyo, all of Japan [73] Assignee: MizusawaKagaku Kogyo Kabushiki Keisha, Osaka, Japan 22 Filed: Aug. 31, 1971 21Appl. No.: 176,743

Related US. Application Data [63] Continuation-impart of Ser. No.775,126, Nov. 12,

1968, Pat. N0. 3,622,364

[52] US. Cl l17/36.2, 117/155 UA, 117/156,

106/288 B [51] Int. Cl B4lm 5/22 [58] Field of Search 117/362; 106/288 B[56] References Cited UNITED STATES PATENTS 3,622,364 11/1971 Sugaharaet a1 106/288 B 3,330,722 7/1967 Amano ..117/36.2 3,293,060 12/1966l-Iarbort 117/362 Primary Examiner-Murray Katz An0rney-Sherman &Shalloway [57] ABSTRACT 2 am 550 V2 URsso) wherein R and R arereflectances of light having wavelengths 430 my, and 550 mp,respectively, when the minerals are developed by benzoyl leucomethyleneblue.

4 Claims, No Drawings PRESSURE SENSITIVE RECORDING PAPER Thisapplication is a continuation-in-part of application Ser. No. 775,126,filed Nov. 12, 1968, now US. Pat. No. 3,622,364.

This invention relates to a pressure sensitive recording paper having asa coating, a color former which demonstrates pronounced colordevelopment effect, such pressure sensitive recording paper beingcapable of reproducing copies by hand writing, printing or typingwithout the necessity of the conventional carbon paper.

The pressure sensitive recording papers, with a few exceptions in thecase of special papers, are in all cases those in which the colordevelopment reaction is attributable to the transfer of electronsbetween the colorless compound of organic coloring matter havingelectron donating properties and a color former, the electron acceptor.(U.S. Pat. No. 2,548,366).

Two classes of coloring matter, each of which exhibits differentbehaviours of coloration, have been used as the colorless compound oforganic coloring matter. One of these is that which, as in the case, forexample, of the triphenyl methane coloring matter, develops colorintensely immediately upon contacting a solid acid, but which has atendency to fade easily (primary color development coloring matter). Thesecond coloring matter used is one which does not immediately developcolor upon contacting a solid acid but develops its color completelyafter several days have elapsed and exhibits adequate fastness tosunlight. The acyl leuco methylene blues, for example, are used as suchcoloring matter (secondary color development coloring matter).

On the other hand, solid acids are generally used as the color formerand the electron acceptor. In the past, materials such as kaolin,bentonite, attapulgite, aluminum sulfate, natural zeolite, silica gel,feldspar, pyrophyllite, halloysite, magnesium trisilicate, zinc sulfate,zinc sulfide, calcium fluoride, calcium citrate as well as the organicacids as tannic acid and benzoic acid have been used.

The pressure sensitive recording paper using these color developmentcoloring matter and color formers is made up of two classes of papers:one, the transfer sheet (referred to as the coated back or C8), a paperwhich has been coated with the coloring matter in solution in oil andencapsulated by using such materials as gelatin, gum arabic or syntheticresin, the size of which capsules is several microns in diameter, andthe other, the receiving sheet (referred to as the coated front or CF),a paper coated with the color former. When such two papers aresuperposed facing each other and pressure is applied with either a steelpen or a typewriter,

the capsules of that portion to which the pressure has been appliedrupture and the oil and the colorless coloring matter come into contactwith the color former to develop color and thus impress that portionwith a mark. When three or more copies are required, one or moreintermediate sheets, which are generally referred to as a coated frontand back sheet (or CFB), i.e., one which has the front coated with thecolor development coloring matter and the back coated with the colorformer, are used interleaved between the transfer sheet and thereceiving sheet.

According to previous proposals, it can be seen that in all cases,research concerning the pressure sensitive recording paper hasemphasized the process of synthesizing the organic coloring matter andencapsulation thereof and practically no studies have been maderegarding the color former of the pressure sensitive recording paper.Accordingly, in the present state of the art, the practice is to useattapulgite, a kind of naturally obtained clay, in its as-obtainedstate.

However, the conventional color formers such as indicated above wereeither those in which, notwithstanding their good color developmenteffect relative to the primary color development coloring matter, theircolor development effect relative to the secondary color developmentcoloring matter was poor, or, notwithstanding their good colordevelopment effect relative to the secondary color development coloringmatter, their color development effect relative to the primary colordevelopment coloring matter was poor. Thus, to date no color formerwhich demonstrates excellent color development effects relative to theprimary as well as secondary color development coloring matter has beenfound.

It is, therefore, a primary object of the present invention to providepressure sensitive recording paper having a color former coated thereonwhich demonstrates excellent color development effects relative to theprimary as well as secondary color development coloring matter.

Still another object of the present invention is to provide suchpressure sensitive recording paper having a color former coated thereonwhich not only excels in its color development effects relative to theprimary as well as secondary color development coloring matter but alsocauses fewer smudges. Smudge, as herein used, refers to a non-intendedcolor development phenomenon (soiling) which occurs, for example, duringpreservation, carrying, handling, etc.

An additional objectof the present inventionis to provide such pressuresensitive recording paper wherein the color former has less exfolidationphenomenon after its application to the paper.

Other objects and advantages of the present invention will becomeapparent from the following description.

The foregoing objects and advantages are achieved according to thepresent invention by a pressure sensitive recording paper to which hasbeen applied a color fonner which comprises a dioctahedralmontmorillonite clay mineral and/or the acid-treated products thereof,the specific surface area of which is at least 180 m' lg. Of the totalmineral particles at least percent by weight have a particle diameter 10microns or less, and not more than 45 percent by weight are those of adiameter of one micronor less. Also, mineral and- /or its acid-treatedproducts have a secondary color development property, K,, of at least1.40, preferably at least 1.60, the value of K, being represented by theformula wherein R and R are reflectances of light having wavelengths 430m p. and 550 m g, respectively, when the mineral and/or its acid treatedproducts are developed by benzoyl leuco methylene blue.

It has been found that in accordance with the present invention that thecolor development effects of the color former in the secondary colordevelopment coloring matter, e.g., acyl leuco methylene blue, iscontrolled to a great extent by the inherent properties of the naturalsolid acid, and that although the color development property of thenatural solid acid relative to the secondary color development coloringmatter can be improved somewhat by such chemical treatments as, forexample, acid, alkali, oxidation and reducing treatments, a substantialimprovement of the color development property cannot be attained. Thesolid acids, which are the color former, which have been known, are thenatural clay minerals, such as kaolin, bentonite, attapulgite andnatural zeolite; however, attapulgite has been principally used.

However, it was found in accordance with the present invention that thesecondary color development property of these natural clay minerals wasvery irregular, there being a marked difference in the color developmentproperty even among those of the same class depending upon such factorsas their locale of production or their position of burial within thesame deposit.

Accordingly, when natural clay minerals, generally referred to as, forexample, attapulgite or bentonite, are used nonselectively, little, ifany, color development property is noted in some instances or the extentof the color dvelopment is not adequate in some instances, and, thus, auniform secondary color development effect is not demonstrated. Inaddition, these natural clay minerals have the shortcoming that theirprimary color development effect is on the whole uniformly poor.

It has been discovered in accordance with the present invention that, ofthe natural clay minerals, dioctahedral montmorillonite clay mineral hasa unique property in that a certain class of the montmorillonite claymineral has, as its inherent property, an excellent color developmentproperty relative to the secondary color development coloring matter(secondary color development property). It has also been discovered thatthis characteristic differs from the other clay minerals in that thecolor development property relative to the primary color developmentcoloring matter (primary color development property) can be enhancedremarkably by an acid treatment. Further, it was confirmed that apractically proportional relationship exists between the primary colordevelopment property of the hereinbefore indicated clay minerals andtheir specific surface area; and that the specific surface area ofdioctahedral montmorillonite clay minerals adopted by the presentinvention is generally increased by its acid treatment though there is adifference in degree depending upon its class and that an enhancement ofthe primary color development property takes place in concomitance withthis increase in specific surface area.

It has been found that a color former for pressure sensitive recordingpaper, which excels both in its primary and secondary color developmentproperty, can be obtained by first choosing from among the dioctahedralmontmorillonite clay minerals those which excel in secondary colordevelopment property and, thereafter, subjecting these chosenmontmorillonite clay minerals to an acid treatment until the desiredspecific surface area is obtained.

Further, it has been found that the color development property of theaforesaid montmorillonite clay minerals can be readily determined bymeasuring, in accordance with the measurement method given below, thereflectances R and R of light having wavelengths 430 m ,u. and 550 m [Lwhen the minerals are developed by benzoyl leuco methylene blue of theformula (CHahN N(CHI):,

and comparing the value of K calculated as follows:

2 um/ 550 sso) Method of Measuring the Secondary Color DevelopmentProperty 1. Preparation of the specimen.

The color former is prepared by winnowing it and comminuting it well ina mortar or pot mill so that of the total particles those of a diameterof 10 microns or less in diameter make up at least percent by weightwhile those of a diameter of one micron or less make up no more than 45percent by weight thereof. Five grams of this powder is then placed in aweighing bottle and is dried in a l 10C. constant temperature dryer forone hour followed by allowing it to cool in a desiccator.

The particle diameter of the color former is measured by means of theAndreasen sedimentation pipet. For particulars, reference is made to theEncyclopedia of Chemical Technology (R.E. Kirk, D. Othmer), Vol. 12, p.490 (1954). The particle diameters used herein have all been made bythis method of measurement. 2. Preparation of the color developmentcoloring matter solution.

Five grams of benzoyl leuco methylene blue are dissolved in grams ofg.p. benzene.

3. Color development conditions.

Two grams of the foregoing dried specimen are weighed onto a watch glass8 cm in diameter and spread out thinly. Four cc. of the benzoyl leucomethylene blue solution are then dropped in such a manner as tocompletely wet the whole specimen, after which stirring is carried outwith a spatula to ensure that the system becomes homogeneous. ifcomplete wetting of the speciman cannot be accomplished by 4 ml. of thesolution owing to the greatness of the oil absorption value of thespecimen, the foregoing operation is carried out after first addingbenzene to the coloring matter solution in a necessary amount. This isfollowed by allowing the specimen to stand for 24 hours in a room of atemperature 15 20C. into which the direct rays of sunlight do not shine.During this time the specimen is mixed two or three times with a spatulato ensure that the entire specimen becomes homogeneous. The specimenobtained is used for measuring the degree of color development.

4. Method of measuring the degree of color development.

The degree of color development of the specimen whose color has beendeveloped in the foregoing manner is measured with a spectrophotometer.The specimen is packed in a powder cell of quartz and its reflectancesof lights having wavelengths of 430 y. and 550 m p. are measured. Thereflectance is expressed in percent using as the basis 100 percent forthe reflectance of an alpha-alumina shaped product. 5. Method ofindicating the secondary color development property.

1. Preparation of the specimen.

After the dried color former has been fully comminuted in a mortar orpot mill, it is winnowed and prepared such that at least 85 percent byweight of the When the felleetanees of the llghts of 430 m P- ahd 5total particles are those having a particle diameter 550 m designatedrespectively 430 and 550 the microns or less and not more than 35percent by weight seeehdal'y Color development P p y defined by arethose of a particle diameter one micron or less. Five the followingequatloh grams of this powder are then weighed into a weighing bottle,dried for one hour in a l 10C. constant temper 2 4ao ss0 sso)- 1O 1 dature dryer and, thereafter, allowed to coo m a esic Thus, the greaterthe value of K the better the secondcaton ary color developmentproperty. For the method of ac- 2 Acid treatment conditions curatelyindicating color, the methods of indication of 8 the Commission onIllumination are used. The determi- SpeclmenS nations of colordevelopment by means of the value of 1S Fift gram Samples of h l on a db i are 2. as defined hel'elnahove. and the results of determlweighedinto six SOO-ml conical beakers. After adding nations by means of thenaked eye have been found 10 300 ml of 16.2 wt. percent hydrochloricacid to each be in very good agreement beaker, they are heated in a 85C.water bath. After the Next, to clarify the fact that the Specificsurface area, passage of each hour one of the beakers is taken out C0101develo To y. Secondary Cole! from the water bath, and the contents arewater- P y development property and decolo z g p p y 0f the washed untilno chloride ion remains, then dried at natural ay minerals differ g ydepending p 110C., comminuted and winnowed to obtain the specisuchconditions as their class, locale of production and Of th 6 i s, th t ter d ted position of production in the same locale and that there on (A)th d t ati th at t decolorizing is achange over a broad range in theseproperties after o rt when a lubricating oil was decolorized at the acidtreatment of the minerals, these properties of 250C, nd (B) thosedemonstrating the greatest dethe various classes of natural clayminerals are shown l i i property h o b n il was decolorized in Table 1,below. at 110C.

TABLE I-(l) Property of untreated specimen Acid treatment I SpccilicPrimary Secondary Dcco1oriz- Specific Primary )ccisurface color dccolor0 1115; surface color de- Class of the clay Name of the clay Localc ofin which clay incn arca vcloprncnt vclopmont ability arca vclopmentmineral mincral produced No. (ml/g.) (K (K2) (percent) (ml/g.) (K1)Attapulglto Attapulgitc Florida ((165%)) 1 96 2.60 1.68 34.4 160 2.

w Georgia 1 2 24 2.06 1.17 39.5 2. 11111 Niigata 3 72 2.20 1.35 29.3 1602. 2s Mica clay mineral Niigata 4 24 2. 71 1. 20 39. 5 24 3.03 Titlemineral... Okayama 5 10 2. 02 1.18 O 10 2. 08

Okayama 6 15 1,74 1.19 1 0 25 1.78 'Isugawa (Nilgata) 7 2. 40 1. 6. 8136 2. 50 lientonite (lII) Gnnma 8 72 2.63 1.84 6.8 2.22 Sub-hcntonitc(l) Mississippi (USA) J 104 2. 40 1.80 5. 7 280 3. 04 DioctahcdralSub-hentonitc (1I) Utah (USA) 10 72 2.48 1.30 1.1 232 2.96montmorillonitc. Japanese acid clay (1)... Nakajo (Niigata) 11 104 2. 452.15 J. 1 200 3.15 Japanese acid clay (ll) do 12 (J6 2.48 1.63 18. 2 2402-70 Japanese acid clay (Ill). Shibata (Niigata)... 13 128 3.10 1.2942.1 3. 43 Japanese acid clay (1V). Tsuruoka (Yamagata). 14 104 2. 42 2.15 33.0 350 3. 21 Japanese acid clay (V) do 15 136 2.19 1. 311 18. 2 1441 2.3

TABLE I-(ii) Property 01 acid-treated specimen (A) Property ofacid-treated specimen (13) Second- Primary Secondary color Dccoler- AcidSpecific color ary color Docolor- Acid dcvelizing trcatsurface devel-(lcvelizing treat- Specmmn opmcnt ability mcnt acid opment opmentability mcnt Suitability for use as color former for pressure No. K2)(percent) time (ml/g.) (K (K2) 1 (percent) time sensitive recordingpaper 1 1.65 52.1 1 160 2.65 1.65 47.0 2 Satiplactory (primary colordevelopment effect 1H CH0! g 2 g 15% 1. 22 54. 2 6 Poor.

1.32 79.7 6 Poor (dccolorizin r0 crt ver' 00d 4. 1.20 39.6 1 32 3.111.20 42.5 5 Poor. g p p y l g 5. 1.16 0 1 1O 2. 19 1. 14 2. 0 4 Do. 6.1.18 0 1 30 1.90 1.15 0 2 a Do. 7. 1. 80 72. 0 1 3. 10 1. 82 73. 2 3Excellent. 8. 1. 82 66. 2 1 120 2. 23 l. 71 70. 2 3 Poor.

2.22 87.1 5 350 3.41 2. 2G 75. 7 0 Excellent (grade become acceptable asa result 01 acid treatment). 1.30 1111.3 2 328 3.03 1.25 80. 3 4 Poor(dccclorizing property very good). 1.80 70.3 2 280 3. 47 1. 72 80. 7 5Exccllcnt gradc bccomc acceptable as a result of acid treatment). 1. 9868. 0 3 340 3. 03 1. 8f! 78. 2 7 Do. 1. 35 64. 7 4 168 3. 69 1. 32 70. 48 Poor. 2.20 70.8 3 400 3.42 1. 57 73.8 6 Exccllcnt grade NU/UJ'.acceptable as a r sult 01 Y n acid trcatmA-nt). 1. 35 68. 1 2 295 3. 201.33 78. f1 4 Poor dccolorizing property a /1d,.

Testing procedures.

The several tests indicated in Table l were conducted in the followingmanner:

2. Specimens Nos. 9-15. 76.5 gram samples of the clay on a dry basiswere weighed into eight SOO-ml conical beakers. After adding 200 ml of34 wt. percent sulfuric acid to each beaker, they are heated in a 85C.water bath. After the passage of each hour, one of the beakers is takenout from the hot water bath and the contents are waterwashed until nosulfate ion remains, followed by drying at 110C., comminution andwinnowing to obtain the specimens. Of these specimens, the tests wereconducted on (A) those demonstrating the greatest decolorizing propertywhen a lubricant was decolorized at 250C. and (B) those demonstratingthe greatest decolorizing property when soybean oil was decolorized at110C. These (A) and (B) specimens are indicated in Table I asacid-treated specimens (A) and (B), respectively.

3. Specific surface area.

The specific surface area of the several specimens were determined bythe BET method which is based on the adsorption of nitrogen gas. Fordetails of this method, reference is made to the following literature:

S. Brunauer, P. H. Emmett, E. Teller,

J. Am. Chem. Soc., 60,309 (1938) The specific surface area, as indicatedherein, has been determined by the foregoing method.

4. Method of measuring the primary color development property.

1. Preparation of the solution of the color development coloring matter.

Crystal voilet lactone, a triphenylmethane coloring matter, is used asthe primary color development coloring'matter. 0.5 Gram of this coloringmatter is dissolved in 99.5 grams of g.p. benzene. The chemicalnomenclature and structural formula of crystal violet lactone are asfollows:

Crystal voilet lactone [3,3-bis(pdimethylaminophenyl)-6-dimethylphthalide CHa 2. Color developmentconditions.

Two grams of the hereinbefore described dried specimen are weighed intoa watch glass 8 cm in diameter and spread out thinly, after which 4 mlof the crystal violet lactone benzene solution are dropped onto thespecimen in such a manner that the entire specimen becomes completelywet. This is followed by mixing the specimen with a spatula to achieve ahomogeneous mixture and allowing the mixture to stand for one hour in aroom temperature of C. into which direct rays of sunlight do not shine.In this way, the specimen is obtained for measuring the colordevelopment. When the oil absorption of the specimen is great andcomplete wetting is not obtained by 4 ml of the solution, the sameoperation is carried out after first having added the necessary amountof benzene to the coloring matter solution.

3. Method of measuring the degree of color development.

The degree of color development of the specimen whose color has beendeveloped under the hereinbefore indicated conditions is measured usinga spectrophotomer. The specimen is packed in a quartz cell having adiameter of 21 mm and a height of 10 mm, and with the width of the slitof 1 mm the reflectances at wavelengths 390, 550, and 590 m p. aremeasured. The reflectance is indicated in precent using as the basispercent for the reflectance of an alphaalumina shaped article.

4. Method of indicating the primary color development property.

When the reflectances at 390, 550, and 590 m p. are respectivelydesignated R R and R the primary color development property, K isdefined by the following equation:

l R390/R590 sso) Thus, it can be said that the larger the value of K thebetter the primary color development property. It has been determinedthat there is very good agreement between the determinations of thecolor development property as expressed by the value of K, and theresults obtained by determinations by means of the naked eye.

5. Method of measuring the secondary color development property.

The method of measuring and indicating this property, as previouslydescribed herein, is used.

6. Decolorizing property.

Fifty grams of unrefined soybean oil are weighed into a hard glass -mltest tube, to which one gram of the specimen is then added. The testtube is then immersed in an oil bath heated at 110C. and the contentsare stirred vigorously for 20 minutes. The specimen is then filteredwith a filter paper, after which the clarified oil is placed in a 20-mmcell. White light is directed against this cell and the lighttransmittance is measured with a photoelectric colorimeter. The lighttransmittance is indicated in percent on the basis of 100 percent forthe light transmittance of distilled water. The decolorizing property ofoil when its light transmittance is T percent is defined by thefollowing equation:

Decolorizing property T 56/100 56 X 100 where 56 percent is the lighttransmittance of the unrefined soybean oil used in the presentexperiment.

7. Criterion of suitability.

Those having a secondary color development prop erty above 1.40 and aspecific surface area above m'lg are considered as being acceptable.

The following facts can be understood from the results of Table l.Namely, 1) the natural clay minerals other than the dioctahedralmontmorillonite clay minerals used in the present invention are allinferior in their primary and secondary color development properties,the only exception being attapulgite (see specimen Nos. 2, 3, and 4 inTable 1). Even though the specific surface area of other minerals isincreased by an acid treatment, there is not much improvement of theirprimary color development property (see specimen No. 3 in Table l).

2. While attapulgite demonstrates considerably good primary andsecondary color development properties in its as-obtained natural state,its specific surface area is not increased to as much as 180- mlg eventhough it is subjected to the acid treatment, and its primary colordevelopment property is also scarcely improved (see specimen No. l inTable I).

3. On the other hand, a certain class of dioctahedral montmorilloniteclays demonstrates good secondary color development property, which whenacid treated increases its specific surface area greatly to alsodemonstrate, concomitantly, a marked improvement in its primary colordevelopment property as well (see specimen Nos. 7, 9, ll, 12, and 14 inTable l).

4. However, certain dioctahedral montmorillonite clay minerals areunsuitable for use as the color fonner of the present invention becausethey are inferior in their secondary color development property althoughthey excel in their specific surface area, primary color developmentproperty and decolorizing property (see specimen Nos. l0, l3, and 15).

5. Further, certain dioctahedral montmorillonite clay minerals excel intheir secondary color development property as in the case withattapulgite but their specific surface area does not increase by meansof the acid treatment and, accordingly, their primary color developmentproperty is not improved (see specimen No. 8 in Table I).

From the foregoing results, it can be understood that the followinggeneral principles hold in the case of the dioctahedral montmorilloniteclay minerals.

a. The secondary color development property being an inherent propertyof the material clay itself cannot essentially be improved though someimprovement can be had by the acid treatment.

b. The primary color development property increases in proportion to anincrease in the specific surface area by means of the acid treatment,the desirable primary color development property (K 2.60) being attainedwhen the specific surface area reaches or exceeds 180 m lg.

c. The decolorizing property is not related at all to the secondarycolor development property.

Therefore, according to the present invention, a dioctahedralmontmorillonite clay mineral having the highest possible K, value, atleast above .1 .40, and preferably above 1.60, is first chosen. Thisclay mineral is then subjected to an acid treatment soas to increase itsspecific surface area to above 180 mlg while ensuring On the other hand,the acid to be used in the acid treatment that is carried out to producethe color former of the pressure sensitive recording paper of thepresent invention may be any inorganic or organic acid which is able toincrease the specific surface area of the montmorillonite clay mineralsto above 180 m lg. However, inorganic acids are generally preferred overthe organic acids for reasons of cost and ease of handling. Of theinorganic acids, sulfuric and hydrochloric acids are particularlyconvenient.

Further, no particularly strict conditions are involved in the acidtreatment. if an acid of dilute concentration is used, either thetreatment time becomes longer or the quantity of the acid requiredbecomes greater. Whereas if the concentration is high, either thetreatment time becomes shorter or the quantity of the acid requiredbecomes less in correspondence to the increase in concentration. Again,if the treatment temperature becomes higher, the treatment time iscorrespondingly shortened. Accordingly, the acid concentration may beconveniently within the range of l-80 percent, but from the standpointof convenience in handling, the acid treatment is preferably carried outat a concentration of the order of 15-45 percent and a temperatureranging between and 105C. In short, in this invention, it isonlyimportant that the acid treatment of the dioctahedralmontmorillonite clay mineral be conducted until its specific surfacearea becomes at least 180 m /g.

However, one thing which must be cautioned against in carrying out theacid treatment is that there are instances in which the secondary colordevelopment property makes a marked decline when the acid treatmentproceeds to an excessive degree. For this reason, it is preferred thatthe acid treatment conditions be so controlled that the specific surfacearea of the clay after treatment comes within the range between 180 m /gand 350 mlg.

Next, Table ll shows the changes in the specific surface area and theprimary and secondary color development properties of acid-treated claydepending upon the degree of acid treatment given.

TABLE II Japanese acid clay A J apancse acid clay B Specific PrimarySecondary Specific Primary Secondary Sulfuric acid surface color decolordcsurface color dccolor dctrcatrnent time area vclopment vclopment areavelopmcnt velopment r.) (mfi/g.) property property (mi/g propertyproperty 0 (untreated clay) 75 2. 45 2.13 62 2. 36 1. 62 0.5 102 2. 2.15 103 2. 42 1. 68 108 2. 2.15 142 2. 50 1.

384 3. 74 1. Pl) 280 3. 48 i. 20

that its K, value does not fall to below 1.40, and preferably not below1.60. Thus, it becomes possible to produce a color former havingextremely satisfactory primary and secondary color developmentproperties. The dioctahedral montmorillonite clay minerals which can beutilized are the natural clay minerals such, for example, bentonite,sub-bentonite, fullers earth, Florida earth, and Japanese acid clay.However, the dioctahedral montmorillonite clay minerals, as used herein,are not limited to only those which have been illustrated.

As shown in Table ll, above, the dioctahedral montmorillonite clayminerals used in the present invention are increased in their specificsurface area by means of the acid treatment, and it can be seen that theprimary color development property becomes good when the specificsurface area becomes greater than mlg. When the specific surface areaincreases in this manner, the capacity to absorb and adsorb oilincreases at the same time. Hence, the acid treatment is necessary andindispensable in the present invention.

It has also been found that the particle size of the color former to beused in the present invention is also a very important factor. That is,when the particle size of the color former becomes too large, there isan increase in the smudging phenomenon, whereas if there is an increasein those particles which are too small, the particles after having beenapplied to paper tend to exfoliate. Moreover, if considered from thestandpoint of the color development effect, a greater color developmenteffect is obtained when the particles of the color former are smaller inboth cases of the primary and secondary color development properties.

It has been discovered that by making the particle size of the colorformer such that the particles of a diameter microns or less are presentin an amount of at least 75 percent by weight, and preferably at least85 percent by weight of the total particles and those of a diameter onemicron or less are present in an amount not exceeding 45 percent byweight, and preferably not exceeding 35 percent by weight of the totalparticles, the color development effect is greatly enhanced to yield anexcellent color former for pressure sensitive recording paper in whichthe undesirable tendency to smudging and exfoliation of the appliedparticles is less.

The relationship between the particle diameter of the color former andits primary and secondary color development properties and therelationship between the particle diameter and smudging are shown inTable 111, below. It becomes apparent from these results that the colorformer should preferably be one containing at least 75 percent by weightof particles of a diameter 10 microns or less to achieve the colordevelopment effect and prevention of the smudging phenomenon.

The specimens submitted to the experiment whose results have beenpresented in Table 111 were those prepared by adjusting the particlesize by winnowing of the acid-treated specimen (B). (No. 12) of thepreviously given Table l.

Method of measuring the soil-resistant property.

One hundred grams of the color former are suspended in 250 ml of water,to which 10 grams of starch are then added. This suspension is appliedto paper of fine quality in an amount such that 7 grams of the colorformer are adhered per square meter of the paper, after which the paperis dried to obtain a coated front sheet. A coated back sheet issuperposed on this coated front sheet, and a stainless steel cylinder 5cm in diameter and weighing 4 kg is then placed on top of the superposedsheets and gently pulled across the sheets. The soiling of the coatedfront sheet which results is measured for its reflectance of light of awavelength 430 m u using a spectrophotometer. The soil-resistantproperty is defined by the following equation:

soil-resistant property reflectance of solid coated frontsheet/reflectance of unsoiled coated from sheet X 100 The greater thenumerical value of the soil-resistant property the less is the soiling.

It is apparent from the results in Table 111 that a marked decrease inthe smudging phenomenon takes place when the content of particles of adiameter 10 microns or less is at least percent by weight, andpreferably at least percent by weight of the total particles. It is alsoseen that the primary and secondary color development effects are alsosuperior with this particle size.

A relationship between the particle diameter of the color former and theexfoliation property of the coated front sheet, such as shown in TableIV, below, is observed. In this experiment, the l.G.T. (lnstrturt voorGrafische Techniek) test was employed for investigating the exfoliationproperty. In this test the tendency to exfoliation is less as the numberbecomes greater. The

specimen used are those whose particle diameter has been adjusted as inTable IV by winnowing the acidtreated specimen (A) (No. 12) of Table 1.

The foregoing l.G.T. test was conducted on the specimen prepared in thefollowing manner in accordance with the below-described measurementmethod. One hundred grams of the color former are suspended in 250 ml ofwater, and then 10 grams of starch are added. This suspension is appliedto high quality paper such that the adhesion of the color former to thepaper amounts to 7 grams per square meter, followed by drying the paperto obtain the coated front sheet. This sheet is submitted to the 1.0.1".test and the rate at which the exfoliation of the color former takesplace is measured. It can be seen that when the content of particles onemicron or less in size exceeds 45 percent by weight the results of thel.G.T. test suddenly become worse.

It can be understood from the foregoing results that according to thepresent invention it is preferred from the standpoint of the colordevelopment effect obtained and smudging that of the total particles atleast 75 percent by weight, and preferably 85 percent by weight, arethose whose particle size is 10 microns or less; and moreover that fromthe standpoint of the exfoliation property of the color former it ispreferred that of the total particles not more than 45 percent byweight, and preferably not more than 35 percent by weight, are those onemicron or less.

This exfoliation property is a considerably important matter in makinggood quality transfer sheets, because if the exfoliation of the colorformer is great a large amount of paste must be used for preventingthis, with the consequence that a marked decline in the colordevelopment effect takes place.

Thus, according to the present invention, success was achieved in theproduction of a color former for pressure sensitive recording paper by aprocedure compris ing choosing from among the dioctahedralmontmorillonite clay minerals one whose secondary color developmentproperty, K value, relative to benzoyl leuco methylene blue as measuredin accordance with the hereinbefore described measurement method, is atleast 1.40, and preferably at least 1.60, subjecting this chosenmontmorillonite clay mineral to an acid treatment to increase itsspecific surface area to at least 180 m /g, and preferably to a value inthe range between 180 m /g and 350 m /g and moreover ensuring that theforegoing secondary color development property, K value does not becomeless than 1.40, and preferably not less than 1.60, followed bywater-washing and drying, and thereafter either comminuting orclassifying the foregoing clay mineral to render it into particle sizesin which, of the total particles, at least 75 percent by weight arethose having a particle diameter microns or less and moreover not morethan 45 percent by weight of the total particles are those one micron orless in diameter.

In producing the color former of the pressure sensitive recording paperof the present invention, the procedure described above need notnecessarily be followed since it is also possible to produce it by thefollowing method.

The color former can also be produced by mixing (A) a dioctahedralmontmorillonite clay mineral which has been acid treated until itsspecific surface area is at least 180 m lg, and preferably at least 220m lg, with (B) a dioctahedral montmorillonite clay mineral or anacid-treated product thereof whose secondary color development property,K has a value of at least 1.40, and preferably at least 1.80 to obtainas a whole a specific surface area of at least 180 mlg and a value for Kof at least 1.40, and comminuting or classifying clay minerals (A) and(B) either before or after their mixture, either separately or at thesame time to render the mixture into particle sizes in which at least 75percent by weight of the total particles are particles having a diameter10 microns or less and moreover not more than 45 percent by weight ofthe total particles are those 1 micron or less in diameter.

When the hereinabove described method of the present invention isfollowed, clay mineral (A) need not necessarily be one whose K, value isat least 1.40. On the other hand, clay mineral (B) or its acid-treatedproduct need not necessarily be one whose specific surface area is atleast 180 mlg.

The color former prepared as hereinbefore described can be applied topaper using the natural or artificial pastes such as, for example,starch, casein, tragacanth gum, CMC, synthetic latex having a bondingproperty, such as styrene-butadiene latex and butadieneacrylonitrilleresin latex and polyvinyl alcohol to make the coated front sheet ofpressure sensitive recording paper. Thus, a good quality coated frontsheet is obtained whose primary and secondary color develop ment effectsduring copying are exceedingly good and moreover in which smudging isheld to a minimum.

Further, to improve the color development effect still further or toincrease the amount of color former added, it is possible to suitablyadd such additives as other natural clay minerals or synthetic inorganicsubstances, e.g., calcium carbonate, silica, silicate; organic orinorganic pigments, e.g., ultramarine, persian blue, chrome yellow, ironoxide, lnda'nthrene, Rhodamine, and Methyl violet; dyestuffs such asfluorescent bleaching agent, e.g., diaminostilbene and benzoimidazole;oxidants, e.g., chloro-anyl, persulfates, dichromates,perhydrochlorides, permanganates, cupric salts, ferric salts, iodine,potassium ferrocyanide and organic acid peroxides; reducing agents,e.g., calcium sulfide and solid organic amines; solid acids, e.g.,alumina, siloca, titania, zinc oxide, zinc chloride, titanium phosphateand zirconium phosphate; and alkaline substances such as sodiumsilicate, sodium pryophosphate and alkaline earth metal hydroxides,e.g., slaked lime.

The present invention will now be illustrated by reference to thefollowing specific examples which are presented for purposes ofillustration only and the present invention is in no way to be deemed aslimited thereby.

EXAMPLES l-S The material clays indicated in Table V were chosen, dried,comminuted and winnowed to prepare them into specimens in which percentof the total weight were particles whose diameter was 10 microns or lessand 30 percent of the total weight were those whose diameter was onemicron or less.

When the test for secondary color development property, as fullydescribed herein, was conducted on the foregoing specimens, the K,values shown in Table V were obtained.

The value of K, of specimen Nos. 1, 2, 4, l0, and l l in Table V wereabove 1.40. Accordingly, the acid treatment of these was carried outunder the following conditions.

Fifty grams on a dry basis of each of the foregoing five classes ofclays were weighed into 500-ml conical breakers and their acid treatmentwas carried out under the conditions indicated in Table V, followed bywater-washing, drying at C., comminuting and winnowing to prepare thespecimens. The particle size of each specimen was in all cases 90percent of particles 10 microns or less in diameter and 25 percent ofthose one micron or less.

TABLE VI Acid treatment conditions Specimen 'lem perti (ll/1011 Ex. N 0,in uture (\vt Time No. Table V Class of acid 0,) percent) (hr.)

1 l Sulfuric acid 85 34 2 2 2 ..tl 85 34 3 4 70 16 3 11 Acetic acid 8527 (i The properties of the several specimens obtained by the foregoingacid treatment are shown in Table V11, below.

specific surface area 310 m /g 1 3.02 K, 1.67

EXAMPLE 8 The acid-treated specimen (B) (specimen No. 11 of Table l) waswinnowed and a specimen having the following particle size was prepared.

92 wt. 26 wt.

10 microns or less 1 micron or less Then 0.25 gram of e.p. sodiumperborate as an oxidant was added to 50 grams of the specimen fol- Asapparent from the foregoing description, when those having a K value ofat least 1.40 are chosen from the various classes of the dioctahedralmontmorillonite clay minerals and these chosen minerals are subjected toan acid treatment under suitable conditions as specified by the presentinvention, minerals which excel in both primary and secondary colordevelopment properties are obtained, and those having particle sizes asspecified by the present invention also excel in soilresistant andexfoliation resistant properties.

EXAMPLE 6 65 percent by weight of the Japanese acid clay specimen B ofTable 11 which had been acid treated for 12 hours and 35 percent byweight of specimen No. 8 of Table l were mixed and then comminuted. Thiscomminuted mixture was then winnowed and a specimen having the followingparticle size was prepared; i.e., 92 percent by weight of particleswhose diameter was 10 microns or less and 23 percent by weight ofparticles whose diameter was one micron or less. When the properties ofthis specimen were tested by the methods fully described in connectionwith Table I, the following results were obtained.

specific surface area 197 mlg l 3.05 K, 1.58

EXAMPLE 7 Specimen 1 (specimen No. 1 of Table l) and specimen 11(Japanese acid clay specimen B of Table 11 which had been acid treatedfor 12 hours) were each winnowed and specimens having the followingparticle sizes were prepared.

Specimen 10 or less 1 or less I 90 wt. 21 wt. 1: ll 93 wt. 24 wt. k

Next, percent by weight of specimen 1 and 80 percent by weight ofspecimen [1 were well mixed, after which the properties of the mixturewere tested by the methods fully described in connection with Table I,with the following results:

lowed by thorough mixing. When this mixture was tested for itsproperties in accordance with the testing methods fully described inconnection with Table l, the following results were obtained.

' specific surface area 280 mlg i 3.40 K, 2.28

EXAMPLE 9 The acid-treated specimen (B) (specimen No. 12 of Table l) waswinnowed and a specimen having the following particle sizes wasprepared.

10 microns or less 1 micron or less Then, 4 grams of slaked lime as analkaline substance were added to grams of the specimen, followed bythorough mixing. When this specimen was tested for its properties by thetest methods fully described in connection with Table l, the resultsobtained were as follows:

specific surface area 340 mlg K1 3.15 K, 1.89

While the present invention has been described primarily with regard tothe foregoing exemplifications, it should be understood that the presentinvention is in no way to be deemed as limited thereto but, rather, mustbe construed as broadly as all or any equivalents thereof.

EXAMPLE 10 and used as comparative examples. The specific surface areasand k and k values of the color formers are shown in Table V111.

.Each of the 5 types of samples was coated on a sheet of paper asfollows, and then color was developed.

50 g. of each same were added to 70 g. of an aqueous solution in which0.5 g. of sodium hexametaphosphate and 0.75 g. of sodium hydroxide hadbeen dissolved, and then the mixture was sufficiently stirred for 1 hourin a 500 rpm. stirring machine to obtain a slurry. The pH of this slurrywas adjusted to 9.5 t 0.3 with a NaOH solution, g. (solid 50 percent) ofSBR (styrene-butadiene rubber) latex (DOW No. 636) were added thereto asan adhesive, and then the mixture was stirred for 15 minutes. The slurrythus prepared was coated on a sheet of paper so that the amount of thecolor former might be 7-9 g/m' with the aid of a coating rod No. 12manufactured by R. D. Specialities Laboratory. This coated paper wasdried at 110C. for 5 minutes in a thermostat, and a so-called CF (coatedfront) papers, on which the color former were coated, were obtained.

On the other hand, sheets of CB (coated back) paper, or sheets of paperon which a predetermined primary color development coloring matter(crystal violet lactone), a predetermined secondary color developmentcoloring matter (benzoyl leucomethylene blue) and a mixture thereof werecoated, respectively, were prepared.

The color formed-coated CF paper and the coloring matter-coated CB paperwere laid one on the other, interposed between a rotary double roller,and a pressure of 500 kg/cm was applied thereto to develop color.

After 24 hours had elapsed, the density of the color developed wasmeasured with a densitometer to examine the color development effect. Inthis case the higher the color density, the larger its value.

The results were as shown in Table VI".

TABLE VIII Specimen No. 11A Ex.6 13A 8A 2A Specific surface area (mlg)200 197 100 100 40 Primary color development (k,) 3.15 3.05 3.43 2.222.38

1 8 Secondary color development (k,) 1.89 1.58 1.35 1.82 1.22 Colordensity Primary 69 68 45 42 1 8 Secondary 22 21 l6 l4 l0 Mixture 72 6850 35 20 From the above results, it is understood that a good colordevelopment effect can be obtained when a color former having a specificsurface area of not less than 180 mlg and a k, value not less than 1.40is coated on paper, and then color is developed under pressure.

We claim:

1. In a pressure sensitive recording paper comprising a paper substrateand a coating of a color former formed thereon, the improvement whereinsaid color former consists essentially of particles of a member selectedfrom the group consisting of acid-treated dioctahedral montmorilloniteclay minerals and mixtures of a major proportion of said minerals with aminor proportion of natural dioctahedral montmorillonite clay minerals,said minerals being characterized by the following: (a) a specificsurface area of at least 180 m'lg; (b) at least 75 percent by weight ofthe total particles being of a particle diameter of 10 microns or less;(c) not more than 45 percent by weight of the total particles being of adiameter of one micron or less; and (d) a secondary color developmentproperty, K,,of at least 1.40, the value of K being represented by theformula wherein R and R are reflectances of light having wavelengths 430mp. and 550 mp, respectively, when the said minerals are developed bybenzoyl leuco methylene blue.

2. The pressure sensitive recording paper of claim 1 wherein saidminerals have a specific surface area within the range of 180 350 m lg.

3. The pressure sensitive recording paper of claim 1 wherein the valueof K is at least 1.60.

4. The pressure sensitive recording paper of claim 1 wherein of thetotal particles of said color former at least percent by weight of saidparticles have a diameter of 10 microns or less and not more than 35percent by weight of said particles have a diameter of 1 micron or less.

2. The pressure sensitive recording paper of claim 1 wherein saidminerals have a specific surface area within the range of 180 - 350m2/g.
 3. The pressure sensitive recording paper of claim 1 wherein thevalue of K2 is at least 1.60.
 4. The pressure sensitive recording paperof claim 1 wherein of the total particles of said color former at least85 percent by weight of said particles have a diameter of 10 microns orless and not more than 35 percent by weight of said particles have adiameter of 1 micron or less.